The frequency accuracy of RC oscillators is limited by the inaccuracy and temperature drift of the frequency setting components. Ideally these should be the resistor R and the capacitor C. However in practice non-ideal behavior of the active components in the oscillator contributes as well. This is especially significant in high speed oscillators and low power oscillators where delay and non-linearity are major contributors to inaccuracy and frequency drift.
The period time (and frequency) of RC based relaxation oscillators is defined by the value of the RC product only. In practice this is not the case for example due to circuit delay and circuit non-linearity. In high accuracy oscillators the contribution of other circuit elements (than the resistor R and capacitor C) is minimized by special circuit topologies.
FIG. 1 shows the principle of a high accuracy RC oscillator of the prior art. The oscillator is built around an integrator and a comparator. For practical reasons, amongst others for making the oscillator circuit less sensitive to small voltage fluctuations caused by noise at the comparators input, the comparator has voltage hysteresis. The reference current source Iref is derived from the reference voltage Vref and a resistor R (Iref=Vref/R). The circuit to create the current is not shown. The output frequency is: Fout=1/RCsw. In FIG. 2 the relevant waveforms are shown. This type of oscillator can be implemented in different ways.
The oscillator frequency produced by the circuit of FIG. 1 is insensitive to comparator delay and other comparator non-idealities. It is therefore very accurate and reliable as compared to other oscillator designs, allowing for a broad field of application in particular in applications requiring high speed and/or low power oscillators.
However, despite the abovementioned accuracy, non-idealities of the comparator are not the only cause of inaccuracy of the frequency. As the market demand for high speed oscillators and low power applications grows, the requirements with respect to accuracy become more and more strict.
A disadvantage of the prior art oscillator design depicted in FIG. 1, is that the frequency accuracy not only depends on the comparator, but also on the rest of the circuit, in particular the integrator. In the oscillator circuit of FIG. 1, the integrator is continuously integrating the current supplied by the current source and once per period the charge at capacitor C is transferred to the integrator capacitor.
The saw-tooth shaped waveform at Vout (see FIG. 2a) shows the slow linear down ramp due to current integration. The rising edge due to charge transfer has zero rise time when the opamp is ideal. However in practice the rise time is limited by the bandwidth and slew rate of the opamp and the rising edge is strongly distorted. During charge transfer the limited bandwidth and slew rate of the opamp cause the integrator to be pushed out of its linear operation mode and the virtual ground voltage at the negative input of the opamp will vary. Therefore the current from the current source (which is not ideal and has a limited output resistance) will vary too and so does the period time and frequency (FIG. 2d shows the real waveforms and the longer period time).
To reduce this effect the charge transfer must be fast which requires a high speed opamp. However such opamp has a high current consumption and makes the circuit not suitable anymore for low current consumption applications.